if that muffler tube is clear, there's some kinda fuel trouble.
One bad thing about working on this bike is you must drop the engine to remove the carburetor. It's not simple or easy..
So, check everything else first.
Check fuel flow. Remove the fuel hose at the carb. Drain a cup of fuel into a clean glass container. You want to see clean fuel flowing thick and steady. Anything else and there's a problem.
Remove the "nut" at the bottom of the petcock. There's a little filter screen in there. Don't try to remove the screen. Just wash it out with carb cleaner or something and/or blow it out with air.
If flow is still restricted, remove the petcock from the tank. Crap collects in there. The internal screen may be broken, and petcock holes are clogged or the screen might clogged.
The fuel tank cap must be vented or a vacuum develops in the tank and flow slows. The cap has a tiny vent hole up on the inside edge.
A cap can be completely rusted internally but look ok outwardly. I've broken one open and it was rusted, clogged and was not venting.
Run with the cap loose or completely off to test ..
Check the choke operation. If it's not opening completely the bike might go about 10 mph .. maybe 20 or more.. but it'll run terrible. A closed choke could foul the spark plug... but it won't make a healthy engine die everytime.
If all else fails.. drop the engine and clean the carb. (Do it exactly right .. or you get punished by having to do it again.)
And fix everything upstream of the carb FIRST or the carb will get dirty and clogged again.
"dropping that engine":https://www.mopedarmy.com/forums/discuss/read.php?f=6&i=48538&t=48537
"some tips on cleaning that carb":https://www.mopedarmy.com/forums/discuss/6/14699/14690/
"idle jet particulars":https://www.mopedarmy.com/forums/discuss/read.php?f=6&i=43698&t=43693
"setting idle mixture and speed":https://www.mopedarmy.com/forums/discuss/6/45465/45463/
"extra credit.. how it meters fuel":https://www.mopedarmy.com/forums/discuss/read.php?f=6&i=21675&t=21097